In the sting operations, run mainly by the Bureau of Alcohol, Tobacco, Firearms and Explosives, undercover agents help a group plan a robbery of a fictional drug stash house, describing large quantities of cocaine for the taking. The defendants are then charged with conspiracy to distribute more than five kilograms of cocaine, even though no actual drugs exist. A conviction carries a mandatory minimum sentence of 10 years, and 20 years if the defendants have previous drug sentences. The defendants also face an additional mandatory five-year sentence on gun-related charges.

In a surprise move, Zachary T. Fardon, United States attorney for the Northern District of Illinois, notified judges and defense lawyers in the past two weeks that prosecutors would seek the dismissal of drug conspiracy charges against 27 defendants in seven of nine continuing stash-house cases. An eighth case did not involve the serious drug charges, and no action has been taken in a ninth.

Most of the defendants are still charged with gun-related crimes that carry a five-year minimum prison term and robbery charges with sentences that are at the discretion of the judge.

“In my experience, it is highly unusual to see the en masse dismissals of the major charge in a big group of similar cases,” said Alison Siegler, the director of the federal criminal justice clinic at the University of Chicago Law School and a co-counsel in four of the cases.

The A.T.F. said the sting operations had put more than 1,000 “violent, hardened criminals” in prison over the past decade.

Federal judges in California and elsewhere have sharply criticized the operations, saying that undercover agents sometimes lured naïve, minor criminals into the conspiracy, then saddled them with mandatory sentences of 15 to 25 years.

Judges in some Chicago cases have allowed defendants to explore whether federal agents engaged in selective prosecution based on race because nearly all the stings took place in largely black or Hispanic neighborhoods.

Mr. Fardon’s office would not comment on the shift, which was first reported by The Chicago Tribune. But legal experts said the decision to dismiss charges raised the possibility of an important change in federal policy, in Chicago if not nationally.

“Dropping the drug charges suggests that the government is trying to make these cases less troubling to judges and others,” said Katharine Tinto, a professor at the Benjamin N. Cardozo School of Law in New York who has studied the stings. “It removes some of the most serious consequences.”

Patrick Rodenbush, a spokesman for the Department of Justice in Washington, said, “This is a decision made by the U.S. attorney in Chicago.” He declined to comment when asked whether senior officials were considering a wider shift against major drug charges in stash-house stings.

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Prosecutor Drops Toughest Charges in Chicago Stings That Used Fake Drugs. Order Reprints | Today’s Paper | Subscribe